Electronic ballasts for gas discharge lamps are often classified into two groups—preheat type and instant start type—according to how the lamps are ignited. In preheat type ballasts, the lamp filaments are initially preheated at a relatively high level (e.g., 7 volts peak) for a limited period of time (e.g., one second or less) before a moderately high voltage (e.g., 500 volts peak) is applied across the lamps in order to ignite the lamps. In instant start ballasts, the lamp filaments are not preheated, so a significantly higher starting voltage (e.g., 1000 volts peak) is required in order to ignite the lamps. It is generally acknowledged that instant start type operation offers certain advantages, such as the ability to ignite the lamps at a lower ambient temperature and greater energy efficiency (i.e., greater light output per watt) due to no expenditure of power on filament heating during normal operation of the lamps. On the other hand, preheat type operation usually results in considerably greater lamp life than instant start type operation.
For many existing preheat type ballasts, a substantial amount of power is unnecessarily expended on heating the lamp filaments during normal operation of the lamps (i.e., after the lamps have ignited). It is thus desirable to have preheat type ballasts in which filament power is substantially reduced or eliminated once the lamps are ignited. Currently, there are at least three known approaches that are directed toward that goal.
In a first approach, which may be termed a “passive” method and which has been commonly employed in so-called “rapid start” ballasts, the filaments are heated via windings on an output transformer that also provides the high voltage for igniting the lamps. A known drawback of this approach is that it is inherently limited as to the degree to which filament heating power may be reduced once the lamps ignite and begin to operate. A detailed discussion of the difficulties inherent in this approach is provided in the “Background of the Invention” section of U.S. Pat. No. 5,998,930, the relevant portions of which are incorporated herein by reference.
A second approach employs a separate filament heating transformer, in combination with one or more electronic switches (e.g., power transistors, such as field-effect transistors), in order to provide preheating of the lamp filaments prior to ignition of the lamps. Once the lamps are ignited, the electronic switches are deactivated, thereby preventing any further heating of the lamp filaments. This approach has been used quite successfully, and has the advantage of completely eliminating any heating of the lamp filaments after lamp ignition. However, this approach has the considerable disadvantage of requiring a considerable amount of additional circuitry (e.g., a filament heating transformer, one or more power transistors, etc.). That fact makes this approach quite costly to implement, especially in the case of ballasts for powering two or more lamps, in which case multiple electronic switches, along with associated circuitry, are typically required.
In a third approach, which is common in so-called “program start” ballasts, an inverter is operated at one frequency (i.e., the preheat frequency) in order to preheat the lamp filaments, then “swept” to another frequency (i.e., the normal operating frequency) in order to ignite and operate the lamps. A common circuit topology for such ballasts includes a voltage-fed inverter (e.g., half-bridge type) and a series resonant output circuit; the series resonant output circuit includes a resonant inductor that commonly includes secondary windings for providing heating of the lamp filaments. This topology has been widely and successfully employed in program start ballasts for powering many common types of lamps. Because this approach is difficult and/or costly to implement in ballasts having self-oscillating type inverters, it is typically employed in ballasts having driven type inverters. More importantly, however, this approach has the significant limitation of not being capable of providing anything that is even close to a complete elimination of filament heating after lamp ignition. This limitation follows from the fact that, for the types of circuitry commonly employed to realize this approach, the ratio of the preheat frequency to the operating frequency is typically limited to be no more than 1.6 or 1.7; consequently, a significant amount of power is still unnecessarily expended upon heating the lamp filaments during normal operation.
What is needed, therefore, is a preheat type ballast in which: (i) the filaments are properly preheated prior to lamp ignition; (ii) little or no power is expended on filament heating during normal operation of the lamps; and (iii) the required circuitry may be realized in a convenient and cost-effective manner. Such a ballast would represent a significant advance over the prior art.
A further problem with existing preheat type ballasts that utilize one or more resonant output circuit(s) is that the effective resonant frequency/frequencies of the resonant output circuit(s) are subject to variation due to a number of factors. This variation may substantially interfere with, among other things, the requirement of generating suitable voltages for properly preheating the filaments of the lamp(s).
As is known to those skilled in the art, the effective resonant frequency of a resonant circuit is dependent upon certain parameters, including the inductance of the resonant inductor and the capacitance of the resonant capacitor. In practice, those parameters are subject to component tolerances, and may vary by a considerable amount. Additionally, the effective resonant frequency of a resonant circuit is also influenced by the lead lengths and/or the nature of the electrical wiring that connects the ballast to the lamp(s); the electrical wiring introduces parasitic capacitances (also referred to as “stray capacitances”) which effectively alter the effective resonant frequency of the resonant circuit(s), and which therefore affect the magnitude of the preheating voltage(s) provided by the ballast to the filaments of the lamp(s). Such parameter variation makes it difficult and/or impractical to pre-specify (i.e., on a priori basis) an operating frequency of the inverter so as to ensure that suitable preheating voltages are provided to the filaments of the lamp(s).
The aforementioned difficulties arising from parameter variation are even more problematic when the ballast includes multiple resonant circuits and/or when the wiring between the ballast output connections and the lamps has a considerable length; in the latter case, the resulting parasitic capacitance becomes a very significant factor. Accordingly, for a given predefined inverter operating frequency, the magnitudes of the filament preheating voltages that are provided by a resonant output circuit may vary considerably, and may, in some instances, prove to be either insufficient or at least considerably less than ideal, for preheating the lamp filaments in a desired manner.
Thus, a further need exists for a ballast that is capable of compensating for parameter variations that affect a resonant output circuit, so as to ensure that the ballast provides an appropriate level of preheating for the lamp filaments. A ballast with such a capability would further represent a considerable advance over the prior art.